Method for preparing a porous thermoplastic resin sheet

ABSTRACT

A METHOD FOR PREPARING A POROOUS THEREMOPLASTIC RESIN SHEET COMPRISING THE PROCESSING STEPS OF: FORMING A SHEET WITH A COMPOSITION CONTAINING THERMOPLASTIC RESIN KNEADED TOGETHER WITH ACID-SOLUBLE INORGANIC PARTICLES AND THEN SOAKING THE SHEET THUS OBTAINED IN A TREATING LIQUID CONSISTING OF ACID AQUEOUS SOLUTION FOR DISSOLUTION OF SAID INORGANIC PARTICLES OUT OF SAID SHEET.

ill-74 XR 3.816.575

3,816,575 METHOD FOR PREPARING A POROUS THERMOPLASTIC RESIN SHEETRinnosuke Susuki, Tokyo, I-Iiroshi Hoshi, Narashino, Jrro Saito, Tokyo,Keiichi Murakami, Sendai, and Humio US. Cl. 260-49 5 Claims ABSTRACT OFTHE DISCLOSURE A method for preparing a orous h sheet comprising theprocessing steps of: forming a sheet With a composition containingthermo kneaded together with acid-soluble inorganic particles and thensoaking the sheet thus 0 tame 1n a rea mg ligu id consisting of acidaqueous solntign for dissolution of said inorganic pgrticlmfsaidsh i-This is a continuation of application Ser. No. 842,3 85, filed July 16,1969, now abandoned.

BACKGROUND OF THE INVENTION The present invention relates to a methodfor preparing a porous thermoplastic resin sheet, and more particularlyrelates to a method for porosifying a thermoplastic resin sheet.

It is well known that a thermoplastic resin sheet can be madegas-permeable and suitable for drawing as well as printing, once it isporosified. The conventional method for preparing a porous thermoplasticresin sheet com- 40 prises the processing steps such as adding avolatile liquid cpispastic such as an aromatic hydrocarbon to a materialresin, or kneading a powdered epispastic such as azobisisobutyronitriletogether with a material resin, and then extruding and stretching saidcomposition so as to define 45 a form of sheet by an extruding method ora calendering method.

In the case of a conventional method as above-described, the producedsheet becomes comparatively high in price, as the epispastic used isvery costly.

Furthermore, the conventional method has some troublesome problems,namely, that when a liquid epispastic as aforestated is utilized, acautious treatment is always needed because of its infiammability and,when a powdered epispastic such as azobisisobutyronitrile is 55utilized, a continuous caution must be exercised against -the poisonoussubstances generated during its vesication.

SUMMARY OF THE INVENTION One of the objects of the present invention isto provide 60 a method for porosifying a thermoplastic polyolefinicresin sheet without utilization of such an epispastic as aforementioned.The present invention forms a porous resin sheet by successiveprocessing steps of kneading fine particles of an acid-soluble inorganicsubstance into ther- 3,816,575 Patented June 11, 1974 moplasticpolyolefin resin, forming a sheet of said composition and then soakingthe thus obtained sheet in an inorganic acid aqueous solution fordissolving out said inorganic particles from said sheet.

As the acid-soluble inorganic particles employed in the presentinvention, there can be enumerated calcium sulfite, calcium sulfate,calcium phosphate, barium carbonate, zinc oxide, etc., as typicalexamples and, further, other inorganic particles can be also utilized,so long as they are soluble in an acid aqueous solution as mentionedhereafter. It is needless to say that, even though it is nonsoluble toacid, an inorganic particle can be kneaded within said resin as afiller.

The quantity of inorganic particles to be kneaded within a thermoplasticresin should be -90% by weight of the resultant composition. The grainsize of the inorganic particles should be determined in consideration ofthe desired porosity for the porous sheet and further should be selectedproperly relative to the quantity to be added,

0 lastic resin 20 and it should be less than 200 microns and morepreferably less than 5 microns.

the acid for use in dissolving the inorganic particle kneaded within theresin, there can be enumerated nitric acid, sulfuric acid, hydrochloricacid, phosphoric acid,

sulfurous acid and mixtures thereof, and these acids are to be used inthe form of aqueous solutions.

In a preferred embodiment of the present invention, calcium sulfite isutilized as the inorganic particles and a hydrochloric acid solution isused as the treating liquid for dissolving out said inorganic particlesfrom said sheet.

Since calcium hydroxide is obtained as a by-product when acetylene isproduced from a raw material of carbide, while sulfur dioxide isobtained as waste gas during petroleum refining processes, calciumsulfite can be pre- 3 pared at a low cost by a reaction between calciumhydroxide and sulfur dioxide.

In an alternative preferred embodiment of the present invention, acationic surface active agent is added to a treating liquid for soakingof the resin sheet prepared by kneading the calcium sulfite particleswith the resin. The existence of a cationic surface active agent in thetreating liquid expedites dissolving out the calcium sulfite particlesfrom said resin sheet and etfectuates their dissolution completely.

In addition, utilization of a cationic surface active agent makes itpossible to dissolve out calcium sulfite particles contained within asheet of a thickn s of more or less than 5 mm Accordingly, a mum can beeasily made gas-permeable just like synthetic leather due to thismethod.

As the cationic surface active agent, it is recommended to use one thatis not soluble within the acid aqueous solution, for example quaternaryammonium halides such as dodecyltrimethylammonium chloride,octadecyldimethylbenzylammonium chloride, dioctadecyldimethylammoni umchloride, etc., or alkylpyridinium halide such as hexadecylpyridiniumchloride, etc.

Since all of these surface active agents are acid-proof, there is noneed for changing the property of the treating liquid as dissolutiondevelops, and a concentration of 0.1 to 1.0% by weight of said surfaceactive agent is most suitable.

Although it is not necessarily impossible to employ a 5 nonionic surfaceactive agent, an anionic surface active agent, an amphoteric surfaceactive agent, they are not good in practical use, because they becomesoluble as a result of co-existence for a long period within theacid.

The dissolution of said inorganic particles can be conducted at normal(room) temperature, however, it is advantageous to heat the treatingliquid up to a higher temperature, because dissolution is acceleratedwithout any damage of the sheet material itself.

When calcium sulfite is utilized as the inorganic particles to bekneaded within the thermoplastic resin, sulfur dioxide aqueous solution,namely sulfurous acid aqueous solution can be used as a treating liquidaccording to the present invention. This aqueous solution is capable ofconverting calcium sulfite to hydrocalcium sulfite which is dissolvedout into said solution.

By adding therein calcium hydroxide, an aqueous solution of the thusobtained hydrocalcium sulfite forms calcium sulfite, and accordinglysaid calcium sulfite can be kneaded together within thermoplastic resinfor recycled utilization.

When a sulfur dioxide aqueous solution is used as a treating liquid, amethod for porosifying a resin sheet, according to the presentinvention, comprises the processing steps of:

(a) forming the thermoplastic resin into a sheet after kneading thereincalcium sulfite including partially recycled calcium sulfite asafore-mentioned,

(b) soaking the thermoplastic resin sheet thus obtained into water andblowing gaseous sulfur dioxide therein for converting calcium sulfitewithin said sheet to hydrocalcium sulfite which is then dissolved outinto the water so as to porosify said sheet,

(c) adding calcium hydroxide to the hydrocalcium sulfite aqueoussolution obtained during step (b) for conversion of hydrocalcium sulfiteto calcium sulfite,

(d) recycling the thus obtained calcium sulfite into step (a).

As described above, it is preferable that calcium sulfite is employed asthe inorganic particles to be kneaded within the thermoplastic resin andhydrochloric acid aqueous solution is used as the treating liquid.However, in this preferred embodiment of the present invention, in somecases it is also preferable in connection with the utilization of thesheet that calcium sulfate particles are employed together with calciumsulfite.

Some of the calcium sulfate, which is not as easily soluble inhydrochloric acid, remains within the sheet even after treatment so asto eliminate the directional stretching characteristic or to contributeto preventing overstretching of the sheet, and, further, makes easy anadjustment of the weight of sheet.

An adequate quantity of calcium sulfite to be used should be less than90% by weight of the mixture consisting of calcium sulfate and calciumsulfite.

Thus, the present invention provides a method for porosifying athermoplastic resin sheet comprising the processing steps of:

(a) blowing gaseous sulfur dioxide into calcium hydroxide aqueoussuspension in order to obtain calcium ,sulfite,

(b) forming a sheet with said resin after kneading calcium sulfiteparticles within the thermoplastic resin, (c) soaking the resin sheetthus obtained in hydrochloric acid aqueous solution so as to dissolveout calcium sulfite particles for porosifying said resin sheet,meanwhile converting the thus dissolved calcium sulfite into the sulfurdioxide and calcium chloride,

(d) collecting sulfur dioxide thus obtained for recycling -in a step (a)and, on the other hand, conducting the reaction between the thusobtained calcium chloride and sulfuric acid in order to obtain calciumsulfate and hydrogen chloride,

(e) kneading the thus obtained calcium sulfate within the thermoplasticresin together with calcium sulfite in the same manner as for the step(b), and causing said hydrogen chloride to be absorbed into thehydrochloric acid aqueous solution of the step (c).

For forming a sheet according to the present invention, conventionalmethods can be employed too. The extrusion method, the calenderingmethod and the inflation method, the latter of which utilizes a die ofthe fishtail type or the manifold type, are well known in general forforming a sheet.

It is also practicable that said resin can be kneaded together with anantistatic agent, a discoloration-preventive agent, plasticizer, desiredpigments and a viscosityimprover, besides the afore-stated acid-solubleinorganic particles.

Since the quantity of inorganic particles to be kneaded withinthermoplastic resin, according to the present invention, iscomparatively large, a rubber, for instance ethylene-propylene rubber,should be preferably added to said resin as a viscosity-improver, so asto make it possible to form a sheet without any difiiculty.

A sheet thus formed is subjected to a treatment for dissolving theinorganic particles, after a conventional process for heat finishing aswell as rolling up. The sheet can be cut to suitable dimensions prior tothe above treatment or after it.

Said treatment for dissolving the inorganic particles is conducted bysoaking the sheet in a treating liquid for 5 to 20 hrs., when the liquidtemperature is normal (room temperature), and for 0.l-2 hrs. when theliquid temperature is C.

However, when a cationic surface active agent is present in the treatingliquid as above-described, the time for soakage can be shortened toone-fifth or so. The acid density in the treating liquid shouldpreferably be 2 to 15%. After being porosified by soakage within atreating liquid, then said sheet is placed in contact with an alkalisolution such as sodium hydroxide aqueous solution for neutralization ofacid fixed upon the surface of said sheet. Then said sheet is dried in aforced draught and there is obtained a porous thermoplastic resin sheetas desired.

A sheet prepared in accordance with the method of the present inventionhas innumerable holes, which are irregularly arranged and succeeding oneanother within said sheet, and a microscopic unevenness is also definedon the surface of the sheet so that the light permeability is quitesmall and on the contrary light reflection is quite large. Further, saidsheet is not expandable as is a conventional sheet, but rather it isfirm like a conventional paper.

Furthermore, the microscopic unevenness of the surface deprives same ofthe brightness, which is customary in thermoplastic resin sheets, sothat said sheet has a soft look and also can be written, drawn orprinted on.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Example 1 Compounding ratio:

Polyethylene (commercially available, prepared at medium low pressure):27% by weight Ethylene-propylene rubber: 2.7% by weight CaSO /2H O(grain size: I 70.3% by weight Thickness of sheet: 0.2 mm. Condition ofacid treatment:

Kind of acid: nitric acid Acid density: 10% by weight aqueous solutionTemperature: about 20 C. Treating time: 20 hrs. Condition ofneutralization:

Alkali density: aqueous solution including 5% by weight of sodiumhydroxide.

As the result of a treatment conducted under the abovementionedconditions, the dissolution of all calcium sulfite particles, which werekneaded within the resin sheet, was thoroughly done so that a sheethaving successive holes measuring 111. in diameter was obtained. Incomparison with an oxide with a thickness of 0.2 mm, the sheet thusobtained has the same gas permeability as the oxide. Furthermore, anexamination of the roughness of its surface by drawing with a pencilwhose hardness was graded as HR, showed that the roughness proved goodfor drawing.

Example 2 Compounding ratio:

Polyethylene (commercially available, prepared at medium low pressure):27% by weight Ethylene-propylene rubber: 2.7% by weight CaSO /zI-I O(grain size: Lu): 70.3% by weight Thickness of sheet: 0.2 mm. Conditionof acid treatment:

Kind of acid: hydrochloric acid Acid density: aqueous solutionTemperature: 100 C.

Treating time: 10 minutes Condition of neutralization:

Alkali density: aqueous solution including 5% by weight of sodiumhydroxide.

As the result of a treatment conducted under the abovementionedconditions, the dissolution of all of calcium sulfite particles, whichwere kneaded within the resin sheet, was done completely so that a sheethaving successive holes measuring 111. in diameter was obtained. Thethus obtained sheet has a gas permeability and surface roughness justthe same as the sheet according to Example 1.

Example 3 Compounding ratio:

Polyethylene (commercially available, prepared at medium low pressure):27% by weight Ethylene-propylene rubber: 2.7% by weight CaSO -2H O(grain size, 5 1.): 70.3% by weight Thickness of sheet: 0.2 mm.Condition of acid treatment:

Kind of acid: nitric acid Acid density: 10% aqueous solutionTemperature: 100 C. Treating time: minutes Condition of neutralization:

Alkali density: aqueous solution including 5% by weight of sodiumhydroxide.

As the result of a treatment conducted under the abovementionedconditions, the dissolution of all calcium sulfate fite particles, whichwere kneaded within the resin sheet, done completely so that a sheethaving successive holes measuring lit in diameter was obtained. The thusobtained sheet proved to be better in gas-permeability than that ofExample 1, and also had the same surface roughness as the sheet ofExample 1.

Example 4 Compounding ratio:

Polyethylene (commercially available, prepared at medium low pressure):25% by weight Ethylene-propylene rubber: 3.0% by weight CaSO -2H O(grain size, 5 1.): 36.0% by weight CaSO /2H;O (grain size, In): 36.0%by weight Thickness of sheet: 0.2 mm. Condition of acid treatment:

Kind of acid: hydrochloric acid Acid density: 10% aqueous solutionTemperature: 100 C. Treating time: 15 minutes Condition ofneutralization:

. Alkali density: aqueous solution including 5% by weight of sodiumhydroxide.

As the result of a treatment conducted under the abovementionedconditions, the dissolution of all calcium sulfite particles and gypsumparticles was sucessfully done so well that a sheet having successiveholes measured 1 to 5; in diameter was obtained.

Example 5 A composition of 29.5 parts by weight of polyethylene preparedat medium low pressure (commercially available) and 70.5 parts by weightof calcium sulfite was processed by a calender method ultilizing atwin-roll testing device (roll diameter, 89 mm., revolution ratio,1:1.2) and thereby was prepared a sheet having thickness of 0.2 mm. Thethus obtained sheet was cut into test pieces with the dimensions of 10cm. x 8 cm. and then was soaked in an acid solution heated to atemperature of 100 C., said acid solution having 0.5% density ofdodecyltrimethylammonium chloride and 10% density of hydrochloric acid.After two minutes soaking, said sheet was picked up from said acidliquid and then was neutralized within an aqueous solution of 5% byweight of sodium hydroxide. Then, after being washed in water, it wasdried so as to obtain a finished product.

The thus prepared product looks just like a Kent paper and is rich intoughness and thereby, is quite suitable for writing or drawing, as itis.

Example 6 A sheet prepared with the same composition as Example 5 wastreated by passing same through an acid solution, a neutralizing liquidand water for washing, all of which had the same composition as thepreceding example. Said sheet was kept heated all through the processwithout being cut in pieces.

In this case, the acid treatment, whose time was adjusted to twominutes, resulted in a product having the same or better properties asthe product of the preceding example.

Example 7 After treating a composition of 30% by weight of polyethylene(commercially available, prepared at medium low pressure), 21% by weightof calcium sulfite and 49% by weight of calcium sulfate by the calendermethod utilizing the same device as Example 5, a sheet thus obtained wascut into pieces with dimensions of 10 cm. x 8 cm. for soaking within theafore-stated solution. After 5 minutes soaking, said pieces of sheetwere removed from said solution and were neutralized in turn with anaqueous solution of 5% by weight of sodium hydroxide. Then they werewashed in water and dried to obtain finished products. The thus preparedproducts are, in nature, very good for writability especially forpencil.

Example 8 By treating a composition of 20 parts by weight ofpolyethylene (commercially available, prepared at medium low pressure)and parts by weight of calcium sulfite in the same manner and utilizingthe same device as Example 7, a product was obtained whosecharacteristics are the same as the product of Example 5.

Example 9 I By treating a composition of 40 parts by weight ofpolyethylene (commercially available, prepared at medium low pressure)and 60 parts by weight of calcium sulfite in the same manner ofprocessing and acid treatment as Example 7, there was obtained a productwhose characteristics are the same as the product of Example 5.

Example 10 (a) Conditions for preparing a polyolefin sheet:

Compounding ratio:

Polyethylene (commercially available, prepared at medium low pressure):27.0% by weight Calcium sulfite (grain size: 1.0-3.0;r): 70.5%

by weight Ethylene-propylene rubber: 2.5% by.weight 7 Utilized rolls:

Diameter of the rolls: 90 mm.; length, 200 mm. Revolution ratio: 1.2Temperature on the surface of rolls :160:5 C. Roll clearance: 0.2 mm.Thickness of sheet, 0.2 mm. Condition for dissolution of calciumsulfite: Dimension of sheet: width, 150 mm.; thickness, 0.2

mm. Reaction tank: 200 x 400 x 200 mm. Quantity of S0, to be blown in:0.4 liter/min. Time for soaking: 20-30 minutes Temperature: l-20 C.Dissolution ratio: 98.5% of calcium sulfite included therein Conditionfor neutralization of hydrocalcium sulfite: Density of hydrocalciumsulfite salvaged: 7% by weight Density of calcium hydroxide: 10% byweight Indicator: phenolphthalein Characteristic of porous polyolefinsheet: Tear resistance: 440 g. Moisture permeability: 750 g./m./24 hrs.

Example 11 Condition for preparing a polyolefin resin sheet: Compoundingratio:

Polyethylene (commercially available, prepared at medium low pressure):15.0% by weight Calcium sulfite (grain size: 1.03.0,u): 82.5%

by weight Ethylene-propylene rubber: 2.5% by weight Utilized rolls:

Diameter of the rolls: 89 mm.; length, 200 mm. Revolution ratio: 1.2Temperature on the surface of rolls: 160:5 C. Roll clearance: 0.2 mm.Thickness of sheet: 0.2 mm. Condition for dissolution of calciumsulfite: Dimension of sheet: width, 150 mm.; thickness, 0.2

mm. Reaction tank: 200 x 400 x 200 mm. Quantity of S0, to be blown in:0.4 liter/min. Time for soaking: 30-40 minutes Temperature: 10-20 C.Dissolution ratio: 99.0% Condition for neutralization of hydrocalciumsulfite: Density of hydrocalcium sulfite salvaged: 7% by weight Densityof calcium hydroxide: 10% by weight Indicator: phenolphthaleinCharacteristic of porous polyolefin resin sheet: Tear resistance (meanvalue): 350 g. Moisture permeability: 820 g./m.=/24 hrs.

Example 12 Preparation of calcium sulfite: Calcium hydroxidezsulfurdioxide=62:5 3 (by weight) Reaction density: 20% by weight of watersuspension Blowing pressure of sulfur dioxide: -10 mm. Hg Reactiontemperature: 80 C. Reaction time: 3 hrs. Condition for formingpolyolefin resin sheet: Utilized roll: twin rolls Diameter, 89 mm.;length 200 mm. Revolution ratio: 16:19 Compounding ratio:

Polyethylene (commercially available, prepared at medium low pressure):27% by weight Styrene-butadiene rubber: 2.5% by weight Calcium sulfite:70.5% by weight Processing temperature: 160:5 C. Processing time: 10-15minutes Roll clearance: 0.2 mm.; thickness of sheet, 0.2 mm. (c)Condition of acid treatment:

Kind of acid and density: 10% by weight of HCl aqueous solution Treatingtemperature: 90 C. Treating time: seconds (d) Characteristic of porouspolyolefin resin sheet:

Tear resistance (mean value): 440 g. Moisture permeability: 750gr./m./24 hrs.

What we claim is:

1. A method'for preparing a porous polyolefin resin sheet, whichcomprises: kneading a polyolefin resin with 20 to 90% by weight ofparticles of a material selected from the group consisting of calciumsulfite, calcium sulfite and mixtures thereof; forming the resultingkneaded mixture into a sheet; and soaking the sheet in an aqueoussolution of an inorganic acid selected from the group consisting ofhydrochloric acid and nitric acid, and thereby dissolving said particlesout of the sheet whereb'y to obtain a porous sheet.

2. A method according to claim 1, wherein said particles are not morethan 200 microns in diameter.

3. A method according to claim 1, wherein said aqueous solution containsfrom about 2 to 15% by'weight of said acid and further contains fromabout 0.1 to 1.0% by weight of a quaternary ammonium halide cationicsurface active agent.

4. A method for preparing a porous polyolefin resin sheet whichcomprises kneading polyolefin resin with 20 to by weight of calciumsulfite particles, forming the resulting kneaded mixture into a sheet,soaking the sheet in an aqueous hydrochloric acid solution therebydissolving calcium sulfite particles out of said sheet to obtain aporous sheet, then adding sulfuric acid into said hydrochloric acidsolution containing calcium sulfite dissolved from said sheet therebyproducing calcium sulfate and hydrogen chloride, then recovering saidcalcium sulfate from said solution and absorbing said hydrogen chlorideinto said hydrochloric acid solution.

5. A method for preparing a porous polyolefin resin sheet whichcomprises kneading a polyolefin resin with 20 to 90% by weight ofcalcium sulfite particles, forming the resulting 'kneaded mixture into asheet, soaking the sheet in an aqueous solution containing sulfurdioxide thereby converting said calcium sulfite to hydrocalcium sulfiteand dissolving same out of said sheet to obtain a porous sheet, thenadding calcium hydroxide into said sulfur dioxide aqueous solutioncontaining hydrocalcium sulfite dissolved from said sheet therebyprecipitating calcium sulfite and then recovering calcium sulfiteparticles from said solution.

References Cited UNITED STATES PATENTS 3,660,551 5/1972 Susuki et al.264-49 3,536,796 10/1970 Rock 264-49 MELVYN I. MARQUIS, Primary ExaminerUS. Cl. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent: No. 3 816575 u Dated June 11 1974 Rinnosuke Susuki, Hiroshi Hoshi, Jiro Saito,Keiichi Murakam and Humio Ito It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Col. 8, line 19; change "sulfite" to ---sulfate--.

Signed and sealed this 29th day of October 1974.

(SEAL) Attest:

McCOY M. GIBSON JR. C. MARSHALL DANN Attesting Officer Commissioner ofPatents FORM PO-1050 (10-69) USCOMM-DC 60376-P59 U45. GOVERNMENTPRINTING OFFICE: 1969 0-366-334.

